1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device having triple wells, and more particularly to a method for fabricating a semiconductor device, in which a triple well accurately defines wells forming a N-well, a P-well, and R-well using an extra ion implantation mask after forming an element isolation oxide film, thereby improving the reliability of an element operation and a process yield in accordance with developing punch characteristics between a well and a well.
2. Discussion of Related Art
The recent trend to fabricate highly integrated semiconductor devices results in forming P- and N-type well regions on a semiconductor substrate and having a triple well structure which forms special well regions in the well regions of different conductive types. For example, it is advantageous that C(complementary)MOS transistors together with P- and N-MOS transistors have a very low power and a fast operative speed.
The method for fabricating a semiconductor device having a conventional triple well structure is described as follows. First, a pad oxide film pattern and a nitride film pattern are formed for isolating elements on a silicon wafer semiconductor substrate, the impurity ions are implanted in the lower portion of the semiconductor substrate exposed by the nitride film pattern in order to form a N-well. Subsequently, an element isolation oxide film is formed on the semiconductor substrate exposed by the nitride film pattern by carrying out a thermal oxidation process, whereas simultaneously N-wells are formed upon diffusing the impurity. High energy ions are implanted and diffused to form a N-well on the semiconductor substrate or a P-well and a R-well in the outer thereof, thereby simultaneously forming P-and R-wells.
As apparent from the above description, when a triple well is formed in the semiconductor device in accordance with the prior art, firstly the ion implantation is performed to form a N-well using a nitride film pattern of an element isolation oxidation mask as a mask, then a P-well and a R-well are formed after diffusing a N-well upon forming an element isolation oxide film.
In the case of performing as above, it is problematic that when the integrated degree of elements increases, a N-well diffused by a thermal oxidation doesn't have effective element isolation characteristics. Furthermore, punch characteristics between a well and a well are weakened, the process yield and the reliability of the element operation are deteriorated since N-well regions aren't defined accurately.